[fix] installation instructions: pip install -e searx (setup.py)

In the past we did not really install searx into the virtualenv.  This is a
problem, since entry-points and other python installation stuff is not trigger.

See discussion: https://github.com/asciimoo/searx/pull/1938#issuecomment-632056508



Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>